1. Field of Invention
This invention relates, in general, to optical processing and to an improvement in Vander Lugt optical correlators. This invention offers significant cost benefits thus allowing an affordable commercial-type product for the first time.
This invention relates particularly to the miniaturization of Vander Lugt optical correlators and more specifically to provide users of personal computers (PCs) with optical processing capabilities. This invention miniaturizes optical correlators to a point where they may be used with or form part of PCs. More specifically, these miniaturized optical correlators are capable of being mounted on plug-in printed circuit boards, or on other mounting means, for desktop PCs of any make or model or capable of being mounted on printed circuit boards, or on other mounting means, for optional external equipment connectable through the ports of desktop or laptop PCs of any make or model.
In addition to their use in or with PCs, miniaturization of optical correlators in accordance with this invention expands their use for other applications as will be apparent from the following description and accompanying drawings.
2. Prior Art
Vander Lugt optical correlators rely on "matched filtering" to accomplish target (object) recognition. Optical correlators recognize objects in a scene by comparing them to a filter, encoded in a hologram. When a match occurs, the correlation shows up as a focused spot of light on an otherwise black background. The location of the spot also corresponds to the location of the object in the scene. Portions of the scene that bear no resemblance to the encoded reference object are filtered out and appear black. Thus, the output of a correlator is a map of values representing the likelihood that an object, or group of features, in the scene at a particular location, is the one of interest, ie, the target.
An optical correlator typically comprises, as shown at 10 in FIG. 1, a first lens L.sub.1, a Fourier transform lens, one focal length from the object plane (scene) 11. A Fourier plane (filter) 12 located one focal length behind the first lens L.sub.1 is optically coupled to a second Fourier transform lens L.sub.2 and the resulting image (correlation) plane 13 is located at the Fourier transform plane of the second lens. Not counting the laser's collimation optics, this is a four focal length, or "4f", system.
Originally, the optical correlator was tailored to a particular application for a particular object or target to be recognized because the filter, such as 12, was specifically designed for that purpose and could not be conveniently changed. However, recent developments in Spatial Light Modulators (SLMs), which alter the properties of a light beam as a function of position across that light beam can function as a programmable and updatable filter allowing a single optical correlator design to meet a number of applications.
FIG. 2 shows a generalized optical correlator 20 utilizing two SLMs 21 and 22 which may or may not be identical. The first input SLM 21 functions to insert an input signal (scene) from an acquisition sensor 23 (not normally considered part of a correlator as indicated by the dashed lines in FIG. 2) into the optical train so that it can be processed. The second SLM 22 functions as a filter which can be updated, or reprogrammed, repeatedly if necessary, according to the object to be recognized. This filter SLM 22 corresponds to the filter 12 in FIG. 1 and the two lens assemblies 24 and 25 in this figure correspond to the two lenses L.sub.1 and L.sub.2 in FIG. 1. The detector (a charged coupled device) CCD 26, located at the image plane, corresponds to the correlation plane 13 in FIG. 1. The output of the CCD 26 is processed in postprocessing electronics 27 according to the information desired.
SLMs are selected according to the type of information being processed and the desired characteristics of the optical processor. There are many types of SLMs; a partial list of which includes Liquid Crystal Televisions (LCTVs), Ferroelectric crystals (FLCs), Holographic optical elements (HOEs), and Acousto-Optic (AO) cells. For this invention, the SLMs selected have relatively small pixels (.apprxeq.30 um pitch) and are reflective. (The SLMs illustrated in FIGS. 1-3 are transmissive). SLMs are devices that modulate a light beam via a separate input signal. These inputs are of two types: optical and electrical. Optical conversion devices (Optically Addressed SLMs) convert an input scene from an external light path to the correlator's laser path by affecting the optical activity of subregions of the device. These devices may be analog (one large single active cell, or pixellated). Electrically addressable devices (EASLMs) use a separate electrical signal (ie, video) to modulate the correlator's laser path by affecting the optical activity of pixels in the modulator.
The input scene contains objects to be recognized, such as a product, a vehicle, or objects in a TV scene, or any 2-dimensional data, to be presented to the correlator 20. The acquisition sensor 23 begins the transference of this real scene to an input image for the correlator at input SLM 21. For example, if an external video system is used, the video signal is then fed into a frame grabber where a computer may preprocess the imagery at video rates, or faster if the SLMs can operate beyond 30 Hz, and this modified video signal is fed into the input SLM 21 in non-interlaced form and allowed to interact with the correlator's laser beam.
FIG. 3 illustrates more clearly what constitutes the acquisition sensor 23 in relationship to the first input SLM 21. Acquisition sensor 23 includes a CCD or equivalent camera 28 with a suitable trigger and perhaps timing signals for the SLM drive electronics 29 so that any view on scene 11 is electronically transferred to the SLM 21. In this case, the SLM is electronically addressable (such as an LCTV) and not optically addressable (such as a light valve). Scene 11 may be staged, or comprise real activities, indoor or outdoors, day or night (if suitable illumination is provided to create a detectable image by camera 28). Polarized light from a monochromatic source (such as a laser diode) 30 is directed through a collimator lens 31 to illuminate the input SLM 21, and is then Fourier transformed by lens 24. It passes through the filter SLM 22 (an electronic holographic optical element), and is Fourier transformed a second time by lens 25, and falls on detector array 26. The output of the detector array is then sent to the postprocessor electronics 27a. The output of the postprocessor 27a may then be returned into the system's main processor 27b, which includes an operator program 27c to run the entire system and a filter generator program 27d to generate and change the filter images in filter SLM 22 and a memory 27e. (Items 27a-e are collectively referred to as postprocessing electronics 27 in FIG. 2.) The operator program and any changes therein and the filter images generated by the filter generator program and any changes may be saved in memory along with the output from the detector array 26. The postprocessor 27a may be contained in the PC. Suitable system synchronization and timing control electronics 32 and drive electronics 33 for the light source 30 are also provided.
Also, while optical correlators were originally restricted to the laboratory because of the extremely stringent alignment requirements, more recently the packaging of the optics into small rugged mounts and the use of other technology, such as folding the light path, compact lens systems, etc, allows miniaturization of optical correlators into stable, reliable and compact units. One such miniaturized correlator using a photographic hologram is only 780 cm.sup.3.
Still more recently, an optical correlator utilized a beamsplitter in a solid block prism assembly to direct the scene's Fourier transform into two filter planes. LCTV technology was used for both its scene and filter SLMs and both were electronically addressed, independently programmable, and operate in an amplitude or phase mode which enabled the use of a variety of types of filters. Both sets of correlations were detected by two CCD cameras simultaneously. Folding the optical path reduced the size of these optics from over 20,000 cm.sup.3 to about 4900 cm.sup.3. [The size of this second system is larger due to the fact that its SLM's resolution elements (pixels) are significantly larger than that of holographic film. The optical path's scale is the square of this dimension.]
Thus far, hardware design has been directed toward reducing the size of optical correlators using the above mentioned optical or electronically addressed SLMs, folded optical paths, laser diodes, CCDs, etc, but no attempt has been made heretofor to miniaturize optical correlators so as to be capable of being mounted on, and formed part of, printed circuit boards to give PC operators optical processing capabilities whether the optical correlator are mounted in the PCs themselves or part of optional external equipment connected to the PCs.